Capillary electrophoresis (CE) instruments use electric fields to separate molecules within narrow-bore capillaries (typically 20-100 μm internal diameter). By applying electrophoresis in a small diameter fused silica capillary column carrying a buffer solution, the sample size requirement is significantly smaller and the speed of separation and resolution can be increased multiple times compared to the slab gel-electrophoresis method. UV absorption and laser induced fluorescence are routinely used as the detection system for CE separation.
Applicant's assignee is the owner of several earlier U.S. patents related to CE systems, see Kensenth et al., U.S. Pat. No. 6,833,919; Kennedy, U.S. Pat. No. 6,833,062; He, U.S. Pat. No. 6,969,452; Kurt, U.S. Pat. No. 7,118,659; and Yeung, U.S. Pat. No. 7,497,937.
CE techniques are employed in numerous applications, including DNA sequencing, nucleotide quantification, mutation/polymorphism analysis, SDS-protein separation, and carbohydrate analysis. In order to improve sample throughput, multiple capillaries or channels were used to perform separations in parallel. For example, in one system a beam expander and a cylindrical lens are used to distribute laser light into a thin line that intersects the axes of the capillaries to provide laser induced fluorescent detection for a multiplexed CE system (K. Ueno et al., Anal. Chem., 66, 1424 (1994)). U.S. Pat. No. 5,582,705 used a laser as the excitation light source for fluorescence detection for a multiplexed CE system, while U.S. Pat. No. 6,788,414 revealed a method to perform UV absorption detection in a multiplexed CE system.
With all of the capillaries or channels illuminated at the same time, scattering, refraction, or reflection of light from neighboring channels will affect the detected channel. That is, detection in one capillary can be influenced by light absorption or fluorescence in the adjacent capillaries, thus affecting trace analysis. This phenomenon is referred to as cross-talk between adjacent capillaries. Cross-talk in the range of 1% to 10% could be observed in the previously mentioned inventions. For accurate analysis, cross-talk needs to be eliminated if possible.
There is, therefore, a need to reduce or eliminate the potentially negative cross-talk effects for trace analyte detection using CE. There are several prior art patented techniques to overcome the cross-talk issue. For example, U.S. Pat. No. 5,274,240 used a mechanical stage to translate the capillary bundle to observe one capillary at a time. U.S. Pat. No. 5,324,401 used individual optical fibers to collect emission light from each capillary to eliminate cross-talk. U.S. Pat. No. 5,790,727 used a waveguide to collect the fluorescent signal to reduce cross-talk. Yet another U.S. Pat. No. 7,340,048 taught use of a mask to block the unwanted scattering light to reduce the cross-talk from the adjacent capillaries. Although these various implementations of different optical design in the hardware to reduce the cross-talk are effective, the cost and the complication of the hardware designs are high. There is, therefore, a continuing need to develop less expensive alternate methods of eliminating cross-talk without increasing instrument complexity or cost. This invention has its primary objective fulfilling this need.